The main goal of the proposed investigation is to elucidate the biochemical mechanisms underlying inactivation of peptide neurotransmitters in the central nervous system (CNS). Although the roles of peptides as chemical messengers in the CNS are beginning to be understood in detail, relatively little is known about peptide inactivation, especially at the cellular level. My study focuses on the bag cells and their target neurons in the abdominal ganglion of the marine snail, Aplysia californica. This system is well suited for the proposed studies because the biochemical and electrophysiological correlates of inactivation can be studied in the isolated ganglion. One bag cell peptide, a-BCP, is rapidly inactivated when released by the bag cells or when arterially perfused through the ganglion. Arterial perfusion mimics the release of a-BCP and allows the inactivation of exogenous a-BCP to be studied in situ under defined conditions. The inactivation of a-BCP can be blocked by arterial perfusion of peptidase inhibitors, suggesting that a-BCP is inactivated by peptidases located in the interstitial and vascular spaces of the CNS. By 1) perfusing and collecting solutions of a-BCP, 2) determining the changes in a-BCP structure by HPLC and amino acid sequence analysis, and 3) assaying the electrophysiological effects of collected perfusates on abdominal ganglion neurons, I will be able to more precisely define the mechanism(s) by which a-BCP is inactivated. Insights gained by these experiments may reveal some of the fundamental mechanisms underlying peptide-mediated neurotransmission.